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Illitization of K-bentonites in the Baltic Basin
(2009-10-29T10:51:55Z) Somelar, Peeter
Mixed-layer illite-smectite is one of most common clays minerals that forms in different geological environments including burial diagenesis, hydrothermal, metasomatic and (contact-)metamorphic alteration. Illitization of smectite is considered to proceed through mixed-layer illite-smectite intermediates, which show a progressive mineralogical trend with an increase of illite at the expense of smectite. This trend termed “smectite illitization” has been used in different ways to provide information about thermal and tectonic history of sedimentary basins. The Ordovician and Silurian sedimentary successions of the Baltic Basin contain numerous altered volcanic ash beds – bentonites that are usually K-rich and can be referred to as K-bentonites. The bentonite clay matrix in the Baltic Basin is typically composed of mixed-layer illite-smectite-vermiculite what can occur with some amount of kaolinite. However, the Upper Ordovician Katian age bentonites are characterized by chlorite-smectite type mixed–layer minerals. The illitization in the Ordovician and Silurian K-bentonite beds in the Baltic Basin is evidently controlled by a combination of burial and fluid–driven processes. The burial process predominated in the deeply buried southern and south-western part of the Baltic Basin. The influence of the burial diagenesis decreases with the decreasing burial depth from the southern part of the basin towards the central part of the basin. We suggest that illitization in the northern and north-western part of the basin was triggered by the prolonged flushing of K-rich fluids in relation to the latest phase of the development of the Scandinavian Caledonides about 420–400 Ma. The K-rich fluids were probably derived by the leaching of the K-feldspar Svecofennian crystalline basement, which were uplifted in the forebulge area of the Caledonian foredeep just at the northern and northwestern margin of the Baltic Basin.